The present invention relates to a Cu—Ga alloy sputtering target that is used for forming a Cu—In—Ga—Se (hereinafter, referred to as CIGS) quaternary system alloy thin film, which is a light-absorbing layer of a thin-film solar cell layer, a method of producing the target, a light-absorbing layer formed from a Cu—Ga based alloy film, and CIGS system solar cell having the light-absorbing layer.
Recently, mass production of a CIGS system solar cell showing a high efficiency as a thin-film based solar cell has been promoted. An evaporation method and a selenization method are known as methods of producing the light-absorbing layer of the CIGS solar cell. The production of solar cells by the evaporation method has an advantage of high conversion efficiency, but has disadvantages of a low deposition rate, a high cost, and a low productivity. Thus, the selenization method is better for industrial mass production.
The outline of the process of the selenization method is as follows. First, a molybdenum electrode layer is formed on a soda-lime glass substrate, and a Cu—Ga layer and an In layer are formed thereon by sputtering, followed by high temperature treatment in a selenium hydride gas to form a CIGS layer. A Cu—Ga target is used in the sputtering of the Cu—Ga layer in the process of forming the CIGS layer by the selenization method.
The conversion efficiency of a CIGS system solar cell is affected not only by various production conditions and characteristics of the constituent materials and the like, but also by the characteristics of the CIGS film to a great extent.
A melting method and a powder method are known as methods of producing the Cu—Ga target. In general, though the Cu—Ga target produced by the melting method contains relatively less impurities, the method has many disadvantages. For example, compositional segregation largely proceeds because a high cooling rate cannot be employed and thereby the composition of the film formed by the sputtering is gradually changed.
In addition, shrinkage cavities tend to occur at the final stage of cooling of the melted metal. The portions surrounding shrinkage cavities have inferior characteristics, and therefore the Cu—Ga targets having shrinkage cavities are unsuitable for processing into a predetermined shape and are not used, resulting in a reduction in yield.
Further, an increase in Ga concentration increases the brittleness and tends to cause cracking, which means that cracking and chipping readily occur during processing into a target or sputtering. This is also a cause of cost increase due to a reduction in yield. Thus, the production of the Cu—Ga target by the melting method is inadequate from the points of cost and characteristics.
A prior document (Patent Literature 1) relating to a Cu—Ga target produced by a melting method describes that compositional segregation is not observed, however, any analytical result is not shown at all. Also, though the document describes that no brittleness is caused and also no cracking is confirmed, it fails to show the processing conditions and sputtering conditions, and the details are unclear.
And the examples only show the results of a Ga concentration range of which upper limit is 30 wt %, and characteristics including brittleness and cracking in a Ga concentration range higher than this upper limit are not described at all.
Meanwhile, the target produced by the powder method generally has disadvantages of a low sintering density and a high impurity concentration, for example. Patent Literature 2 relating to a Cu—Ga target describes about a sintered compact target, wherein brittleness in conventional technology, i.e., tendency of occurrence of cracking and chipping during cutting out of targets, is described, and this problem is solved by sintering a mixture of two types of powder.
One of the two types of powder is a powder containing Ga in a high content, and the other is a powder containing Ga in a low content. The powders generate a two-phase coexisting structure surrounding a grain boundary phase.
This process includes production of two types of powder and is therefore complicated. In addition, since the powders have different physical properties such as hardness and textures, it is difficult to obtain a uniform sintered compact by simply mixing and sintering only, and an improvement in relative density cannot be expected.
A target having low density obviously causes abnormal discharge and generation of particles. Deformed materials such as particles present on a sputtering film surface adversely affect the characteristics of the subsequently formed CIGS film and may ultimately cause a considerable reduction in the conversion efficiency of the CIGS solar cell with high probability.    Patent Literature 1: Japanese Patent Application Laid-Open Publication No. 2000-73163    Patent Literature 2: Japanese Patent Application Laid-Open Publication No. 2008-138232